The present invention relates to a fail-safe control of an electronically controlled automatic transmission to be mounted on a vehicle, and, more particularly, to a fail-safe control against failures in the mechanic portion of a transmission (T/M) such as stick valves or slippages of the friction materials or the like.
In the conventional electronically controlled automatic transmission, the signals corresponding to the degree of throttle and the car speed or the output rotational speed of the T/M are used to judge the gear ratio of the T/M and whether the lock-up clutch is turned on or off to control an actuator such as a solenoid installed in the T/M is controlled.
However, in the above-described type of conventional control method, while there is no problem in fail-safe control against electric failures of the sensors or the actuators, it is difficult to perform a sufficient fail-safe control against failures in the mechanical portions of the T/M, for example, stick valves or the slippages of friction materials. Therefore, a further improvement is required for comfortable driving.